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Mutational and secondary structural analysis of the basolateral sorting signal of the polymeric immunoglobulin receptor.
The 17-juxtamembrane cytoplasmic residues of the polymeric immunoglobulin receptor contain an autonomous basolateral targeting signal that does not mediate rapid endocytosis (Casanova, J. E., G. Apodaca, and K. E. Mostov. Cell. 66:65-75). Alanine-scanning mutagenesis identifies three residues in this region, His656, Arg657, and Val660, that are most essential for basolateral sorting and two residues, Arg655 and Tyr668, that play a lesser role in this process. Progressive truncations suggested that Ser664 and Ile665 might also play a role in basolateral sorting. However, mutation of these residues to Ala or internal deletions of these residues did not affect basolateral sorting, indicating that these residues are probably not required for basolateral sorting. Two-dimensional NMR spectroscopy of a peptide corresponding to the 17-mer signal indicates that the sequence Arg658-Asn-Val-Asp661 has a propensity to adopt a beta-turn in solution. Residues COOH-terminal to the beta-turn (Arg662 to Arg669) seem to take up a nascent helix structure in solution. Substitution of Val660 with Ala destabilizes the turn, while mutation of Arg657 to Ala does not appear to affect the turn structure. Neither mutation detectably altered the stability of the nascent helix in the COOH-terminal portion of the peptide
Characterization of entangling properties of quantum measurement via two-mode quantum detector tomography using coherent state probes
Entangled measurement is a crucial tool in quantum technology. We propose a
new entanglement measure of multi-mode detection, which estimates the amount of
entanglement that can be created in a measurement. To illustrate the proposed
measure, we perform quantum tomography of a two-mode detector that is comprised
of two superconducting nanowire single photon detectors. Our method utilizes
coherent states as probe states, which can be easily prepared with accuracy.
Our work shows that a separable state such as a coherent state is enough to
characterize a potentially entangled detector. We investigate the entangling
capability of the detector in various settings. Our proposed measure verifies
that the detector makes an entangled measurement under certain conditions, and
reveals the nature of the entangling properties of the detector. Since the
precise characterization of a detector is essential for applications in quantum
information technology, the experimental reconstruction of detector properties
along with the proposed measure will be key features in future quantum
information processing.Comment: 18 pages, 6 figure
Spatial and temporal characterization of a Bessel beam produced using a conical mirror
We experimentally analyze a Bessel beam produced with a conical mirror,
paying particular attention to its superluminal and diffraction-free
properties. We spatially characterized the beam in the radial and on-axis
dimensions, and verified that the central peak does not spread over a
propagation distance of 73 cm. In addition, we measured the superluminal phase
and group velocities of the beam in free space. Both spatial and temporal
measurements show good agreement with the theoretical predictions.Comment: 5 pages, 6 figure
Stationary distributions of continuous-time Markov chains: a review of theory and truncation-based approximations
Computing the stationary distributions of a continuous-time Markov chain involves solving a set of linear equations. In most cases of interest, the number of equations is infinite or too large, and cannot be solved analytically or numerically. Several approximation schemes overcome this issue by truncating the state space to a manageable size. In this review, we first give a comprehensive theoretical account of the stationary distributions and their relation to the long-term behaviour of the Markov chain, which is readily accessible to non-experts and free of irreducibility assumptions made in standard texts. We then review truncation-based approximation schemes paying particular attention to their convergence and to the errors they introduce, and we illustrate their performance with an example of a stochastic reaction network of relevance in biology and chemistry. We conclude by elaborating on computational trade-offs associated with error control and some open questions
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